1. Field of the Invention
The present invention relates to an biological luminal body evaluating Apparatus for evaluating a property or organ of a luminal body inside a living body.
2. Background Art
It is well known that objective measurement and evaluation of flexibility of an artery, vein, or other luminal body in a living body by non-invasive measurement are effective in some cases. They are effective upon successively or continuously evaluating, for example, a degree of progress of arteriosclerosis and for providing information for thereby applying treatment before serious symptoms occur. The serious symptoms include myocardial infarction, vascular cerebral infarction, arteriosclerosis obliterans, and aneurysm, etc.
The following methods have been known for evaluating elasticity of a vascular wall. In a first method, with a pulse wave velocity PWV (=L/DT) measured based on a time difference DT of a pulse wave between two positions along an artery separated by just a predetermined distance L, arteriosclerosis is evaluated using the pulse wave velocity PWV. In a second method, a vascular diameter Ds under a systolic blood pressure (maximum blood pressure value) Ps, and a vascular diameter Dd under a diastolic blood pressure (minimum blood pressure value) Pd are respectively measured, for example, during a single heart pulse. Thereafter, a stiffness parameter β [=In (Ps/Pd)÷(Ds−Dd)/Dd] is calculated to evaluate arteriosclerosis using it. Non-Patent Document 1 and Non-Patent Document 2 disclose such second method.                Non-Patent Document 1: “Clinical Studies of Arterial Waves,” Apr. 10, 2003, published by Medical Review Co., Ltd., pp. 94-95.        Non-Patent Document 2: “Medical Technology,” Jan. 15, 2006, published by Ishiyaku Publishers, Inc., pp. 35-40.        
Meanwhile, in order to measure across a wider pressure range, there has been proposed a method where a measured portion of a living body is compressed using a bag filled with water, and a pressure (transmural pressure) applied to a vascular wall is defined as a difference between the pressure of compression and a blood pressure value. Elastic characteristic of the vascular wall is measured based on a variation of a vascular diameter with varying the transmural pressure. Non-Patent Document 3 discloses such blood vessel evaluating method. With this evaluating method, in accordance with a physiological pressure range or pressurization of the vascular wall during measurement, a range of an internal/external differential pressure of the vascular wall, i.e. a transmural pressure PA (=artery internal pressure−artery external pressure) is expanded as follow. It expands from a pressure range having a diastolic pressure as a lower limit value and a systolic pressure as an upper limit value, to a state in which the lower limit value is lower than the diastolic pressure. The elastic characteristics of a blood vessel can thus be known within the expanded range.                Non-Patent Document 3: In Vivo Human Brachial Artery Elastic Mechanics; Alan J. Bank et al; Circulation 1999; vol. 100; 41-47        
However, the conventional method for measuring the elastic characteristics of a blood vessel has a disadvantage that the elastic characteristics of the blood vessel can be known only within the pressure range in which the upper limit value of the transmural pressure PA is the systolic pressure. In general, the elastic characteristic of the blood vessel is nonlinear. As increase of the blood pressure i.e. the transmural pressure PA, the variation of the vascular diameter D relative to the blood pressure variation decreases rapidly, which appears prominently in the arteriosclerosis.
In particular, in cases of hardening of a blood vessel wall due to arteriosclerosis etc. accompanying aging, a characteristic of the variation of the vascular diameter relative to the blood pressure variation decreasing sharply in a region of comparatively high blood pressure value, is manifested. For this reason, in order to accurately know the variation of blood vessel elasticity for diagnosis and prevention, the blood vessel elastic characteristic in the high pressure region exceeding the high blood pressure value i.e. the upper limit value of the transmural pressure PA is desirably measured to be used in the diagnosis. However, the conventional method described in Non-Patent Document 3, disenabling to know the elastic characteristics in the high pressure region of transmural pressure no less than the systolic pressure, cannot not be used to know an adequate precision in regard to the elastic characteristics of a luminal body. Thus, it can not provide the adequate diagnostic precision, for example, in the arteriosclerosis.
FIG. 13 shows relationships between the transmural pressure PA and a compliance CC indicating flexibility of an artery, for a normal subject NAD, a patient I with mild arteriosclerosis, a patient II with moderate arteriosclerosis, and a patient III with severe arteriosclerosis. With the mild arteriosclerosis patient I, the compliance near 100 mHg increases once and then decreases, locally exceeding that of the normal subject NAD. The compliance decreases continuously in the high-pressure region. That is, even if the variation does not appear near 100 mmHg, it appears in the high-pressure region of 150 mmHg or more. For this reason, the conventional method can not provide the adequate diagnostic precision for arteriosclerosis.                Non-Patent Document 4: Biorheology; 1984; 21(5): 723-34. Richter H A, Mitteremayer C: Volume, elasticity, modulus of elasticity and compliance of normal and arterotic human aorta.        